The progression towards autoimmune disease is a complex process involving genetic factors and the long term destruction of specific organs by the infiltration of cells of the immune system, including T cells. In a milestone in immunotherapy, Shapiro and colleagues achieved successful islet cell transplantation in patients with Type I diabetes treated with combination therapy including rapamycin where autoimmune destruction of islet cells was prevented. Given this, we are about to embark on a Phase 1/11,open-label, randomized, pilot trial to evaluate the safety and efficacy of Rapamune(r) in patients with MS and this trial has been approved by the Autoimmunity Centers for Excellence. Our laboratory has created an in vitro model of the effects of exposure to rapamycin on CD4+ versus CD8+ T cells. Surprisingly, while rapamycin blocked T cell proliferation at low strengths of signals, strong signals delivered through the immune synapse resulted in higher proliferative response in the presence of rapamycin. Rapamycin-resistant proliferation was associated with bcl-xL induction and p27kip degradation. We propose to extend our in vitro observations to determine the in vivo mechanism of drug action in patients undergoing immunosuppression with rapamycin. We will first examine the in vivo effects of rapamycin therapy in patients with MS to validate our in vitro model of rapamycin exposure. Then, we will examine the array of gene expression induced by in vivo rapamycin therapy. These studies may reveal that only a subset of patients respond to treatment, and these responses are associated with a particular immune response profile. Such data would be critical in determining which immune phenotypes are correlated with response to therapy. Thus, we believe that determining the immunologic effects of rapamycin in patients with MS in relationship to both specific immune abnormalities associated with the disease and therapeutic responses is critical in probing the underlying pathophysiology of the disease. Elucidating the molecular details and in vitro correlates of the effects of in vivo rapamycin treatment may translate into the development of unique combination therapies designed to prevent the development of, or treat, MS.